One of the most difficult concepts to explain in biology is certainly life itself. But I am not here today to talk about the definition of life, but rather of another puzzling concept: behavior.

Behavior is the central subject of ethology and psychology, but it is also something more commonly understood by personal intuition, just like life, but no formal and widely accepted definition exists.

The simplest definition would be that behavior is something that is done. But in that case we fall into another difficult concept, the concept of “doing”, because what exactly is doing something?

Nobody doubts that a spider building a web is a behavior. Photo by Hedwig Storch.*

Some definitions of behavior that have been published are the following:

Tinbergen (1955): “The total movements made by the intact animal”. According to this definition, only animals can behave, so an unicellular alga swimming towards light, or a plant closing its leaves after being touched cannot be considered behaviors. On the other hand, the fact that an animal is orbiting the Sun because it is on Earth could be a behavior.

Beck et al. (1991): “External visible activity of an animal, in which a coordinated pattern of sensory, motor and associated neural activity responds to changing external or internal conditions”. Here again only animals would behave and only animals with some sort of nervous system. A behavior needs to include a response to a changing condition, i.e., a stimulus.

Starr & Taggart (1992): “A response to external and internal stimuli, following integration of sensory, neural, endocrine and effector components. Behavior has a genetic basis, hence is subject to natural selection, and it commonly can be modified through experience.” This definition does not use the word “animal”, but includes the need for neural components, which is almost the same thing.

Wallace et al. (1991): “Observable activity of an organism; anything an organism does that involves action and/or response to stimulation”. A more simple and broad explanation that encompasses many things that the previous definitions would exclude, but still includes at least the criterion that it is a response to stimuli.

Raven & Johnson (1898): “Behavior can be defined as the way an organism respond to stimulation”. A definition similar to the previous one.

Davis (1966): “What an animal does”. Very ambiguous and contradictory, as the same book includes a section on behavior of plants.

Grier & Burk (1992): “All observable or otherwise measurable muscular and secretory responses (or lack thereof in some cases) and related phenomena such as changes in blood flow and surface pigments in response to changes in an animal’s internal and external environment.” Another confuse, long, complex and ambiguous definition.

When a venus flycatcher closes its leaf to capture a fly, is it behaving? Photo by Stefano Zucchinali.*

Trying to find a way to create a unified definition of what is behavior, a group of researchers from the University of California, Berkeley, made a survey, published in 2009, in which they presented two lists to several biologists. The first contained a series of statements regarding behavior and the respondents should agree or disagree with the statement based on their assumption of what is behavior. The 13 statements were:

(A) ‘A developmental change is usually not a behavior.’ (B) ‘Behavior is always a response to the external environment.’
(C)’A behavior is always an action, rather than a lack of action.’
(D) ‘All behaviors are directly observable, recordable and measurable.’ (E) ‘People can all tell what is and isn’t behavior, just by looking at it.’ (F) ‘Behavior is always influenced by the internal processes of the individual.’ (G) ‘Behavior always involves movement.’ (H) ‘Behaviors are always the actions of individuals, not groups.’ (I) ‘Behavior is something whole individuals do, not organs or parts that make up an individual.’ (J) ‘A behavior is always in response to a stimulus or set of stimuli, but the stimulus can be either internal or external.’ (K) ‘Behavior is something only animals (including people) do, but not other organisms.’ (L) ‘In humans, anything that is not under conscious control is not behavior.’ (M) ‘Behavior is always executed through muscular activity.’

An arctic fox changing its fur color between seasons is a behavior or not? Photo by Wikimedia user Longdistancer.*

The second list included a set of 20 phenomena and the respondents should say whether they considered each phenomenon as representing a behavior or not. (In parentheses are the above statements under which the phenomenon would not be considered a behavior).

Ants that are physiologically capable of laying eggs do not do so because they are not queens. (C, G).

A sponge pumps water to gather food (B, M).

A spider builds a web.

A rabbit grows thicker fur in the winter (A, G, I, M).

A plant’s stomata (respiration pores) close to conserve water (I, K, M) .

A plant bends its leaves towards a light source (K, M).

A person’s heart beats harder after a nightmare (B, I, L).

A person sweats in response to hot air (G, I, L, M).

A beetle is swept away by a strong current (F, M).

A rat has a dislike for salty food (B, C, G, J, M).

A person decides not to do anything tomorrow if it rains (B, C, G, J, M).

A horse becomes arthritic with age (A, B, E, G, M).

A mouse floats in zero gravity in outer space (E, F, G, M).

A group of unicellular algae swim towards water with a higher concentration of nutrients (F, H, K, M).

A frog orbits the Sun along with the rest of the Earth (F, M).

Flocks of geese fly in V formations (H).

A dog salivates in anticipation of feeding time (B, G, I, M).

Herds of zebras break up during the breeding season and reform afterwards (H).

A chameleon changes color in response to sunlight (G, M).

A cat produces insulin because of excess sugar in her blood (B, G, I, M).

Considering the phenomena, seven (2, 3, 11, 14, 16, 17, 18) met the criterion for approval as behaviors based on the results of the statements and seven (4, 8, 9, 12, 13, 15, 20) met the criterion for rejection. The remaining six phenomena (1, 5, 6, 7, 10, 19) had major divergences as to whether they were behaviors.

Several respondents contradicted themselves. For example, many of them agreed that only animals can behave (statement K) but also considered that algae swimming towards water with higher concentration of nutrients is a behavior (phenomenon 14).

Most people would not consider that dandelion fruits carried by the wind are behaving.

Despite the high rate of disagreement, the group decided to propose a definition of behavior. And it is:

The question is not settled, though, and probably never will. Later, Dr. Raymond M. Berger, discussing the same subject, tells us that under the view of Descriptive Psychology, a behavior always includes eight parameters in the following formulation:

<B> = <I, W, K, K-H, P, A, PC, S>, in which:

B = behavior (e.g., Mary playing her queen of hearts in the contexts of a game of bridge).
I = identity of the person whose behavior it is (e.g., Mary)
W = want, the thing the person is attempting to achieve (e.g, to win a trick in the bridge game).
K = know, the cognitive parameter, the knowledge of how things work (e.g., queen vs. king, hearts vs. diamonds).
K-H = know-how, the ability to do what is being done (e.g., the ability to understand the rules of bridge, or the ability to move physical objects).
P = performance, the bodily processes involved in the behavior (e.g., Mary grasping and laying down her queen).
A = achievement, the outcome of the behavior (e.g., Mary takes the trick).
PC = personal characteristics, the indivudual’s difference parameter (e.g., Mary’s deep knowledge of strategy).
S = significance, what the behavior mean (e.g., Mary is playing bridge).

Playing a cardgame is certainly a behavior. A very complex one.

Such a “definition” is, in my opinion, too complex for most circumstances of animal behavior. However, I’m not completely happy with the definition by Levitis et al. either. I think it is hard to tell the difference between a response given by the whole organism vs. one of its parts. For example, when I cough because water entered my trachea, is my whole organism responding or only part of it? Would that be a behavior?

Also, I’m not sure whether we should really consider developmental changes as something different from other responses. If I had to define behavior, I most likely would say it is:

“An activity performed by an organism that is a response to a stimulus and is dependent on the organism’s internal processes”.

Why should a pupil changing size according to the environmental light not be considered a behavior?